Humanitarian land nine removal (that is complete manual minefield clearing) involves very time consuming and dangerous human excavation of a field. Currently a simple hand prodder with a short metal probe is used to investigate surface soils "by feel" for objects. If an object is encountered by the prod having a characteristic size and depth, it must be carefully excavated to determine whether it is a mine. In the case of a mine, it will be defused or detonated. In practice, however, many objects are painstakingly excavated which are not mines, but rocks, pieces of metal or other material. Metal detectors may be used to locate metal objects, however, many mines are not metal, but plastic or even wood.
Minimum metal content (MMC) mine detectors having a search head and circuitry for detecting buried non-metallic and metallic land mines are well known. For example, U.S. Pat. No. 4,016,486 in the name of Pecori assigned to the United States of America by the Secretary of the Army, discloses such circuitry. An MMC detector includes electronic circuitry to assist a human operator to determine the nature of a solid obstruction encountered below the surface of the ground. Typically, metals and rocks are distinguished from one another. Metals are potential land mines. A prodder capable of distinguishing threats, from non-threats reduces stress and fatigue of a human operator and speeds up the process of clearing an area of buried land mines. The search head is typically a UHF balanced bridge detector which is unbalanced by passing the search head over a soil area which has a dielectric constant different from the background. Such a condition exists when passing over a mine.
Currently, instrumented prodders are known having ultrasonic means in the form of an ultrasonic transducer at or near the probe tip that are used for characterization of buried obstructions; this device can be used in conjunction with an MMC detector wherein the MMC detector first detects the presence of metal within or about the ground indicating the vicinity of a land mine, and, wherein the instrumented prodder is used to probe the earth in the vicinity of the suspected land mine, the location of which may have been isolated using the MMC detector.
A hand held prodder having a probe in the form of an elongate, preferably non-magnetic rod including a gripping handle disposed at one end is currently known. The design of the probe is based partially upon a Split Hopkinson Pressure Bar (SHPB) apparatus. In the apparatus a compression wave or high frequency elastic mechanical pulse is delivered via a to a sample wherein a portion of the wave is reflected. Mechanical impedance is a characteristic of a material. An incident wave launched at a material will be reflected and/or transmitted from or through the material, in dependence upon the characteristics of the material. The effect of mechanical impedance on a SHPB apparatus in three instances is described hereafter:
Firstly and obviously, if the mechanical impedance of a sample under test is the same as that of an incident bar in the SHPB, there will be no reflection as the sample will be displaced in a same manner as the bar itself as the compression wave is delivered. The displacement of the end of the bar is directly proportional to the strain measured (.epsilon.).
Secondly when the mechanical impedance of a sample is considerably greater than that of the bar, a sample's mechanical impedance tends toward being infinite and substantially the entire wave is reflected.
In a third instance when the mechanical impedance is zero, in the absence of a sample, the reflected wave is tensile but of equal magnitude to the incident wave. The phase of the wave is shifted by .pi. and the net stress is zero; the relative displacement at the bar end equals twice that for the first instance (2.epsilon.).
In a SHPB device, once the relative displacement of the bars is known, the displacement of the sample is ascertained. Taking into account Young's Modulus (E) and the displacement of the bar, the imposed stress can be calculated, wherein the force applied is equal to the product of the stress and the cross-sectional area of the bar.
Since the loading on the sample becomes equal after a short time, the analysis may be somewhat simplified. Strain results may be used for only the incident bar; or alternatively, the striker bar may be directed to impact directly on the sample, and the transmitter bar alone may be used to define the sample characteristics.
It has been found that plastics, minerals and metals may be discerned from one another by using this approach.
It has been further found that a hand held prodder having a rod modified to be analogous to the incident bar of a SHPB may be used to detect or discern metal, plastic and rocks.
The prodder rod is provided with one or more piezoelectric transducers capable of generating an acoustic wave into the rod and for detecting reflected waves from an object contacting the end of the rod. Conveniently, signal processing means are coupled to the transducers and are provided for analyzing the detected reflected waves for determining the characteristics of the object; more especially distinguishing landmines from inert rocks. The signal processor establishes measurements of the frequency-time-amplitude characteristic of the object. The reflected waves are compared with known characteristic signatures of a plurality of materials to attempt to ascertain a match within predetermined limits.
Although instrumented prodders of this type may function satisfactorily in many instances, they suffer from a problem related to the fact that acoustic coupling at the obstruction is a function of the applied force to the probe end.
Preferably, enough force will be applied to the probe end such that characterization of the obstruction can occur without causing detonation; and, preferably, a relatively consistent force will be applied to the probe end such that an accurate determination as to the character of the buried obstruction can be made. However, if too little force is applied at the probe end, a poor reading may result and a mine in the vicinity of the probe may go undetected. Too much force applied at the probe end in the vicinity of a land mine may inadvertently detonate the mine.
The critical feature for reliable performance of the acoustic probe is the coupling between the acoustic transducer and the transmitting rod probe. To transmit waves without distortion, the probe must be free of imperfections such as interruptions (air pockets) or resonance impeding contacts (such as screws or welds) which would dampen the transmission. Electrical contact likewise will affect the transmission. Thus it is necessary to couple the probe to the transducer with sufficient rigidity to withstand use as a manual tool without the use of screws, welds or other structural discontinuities, and further to operatively couple the piezoelectric transducer to the probe without electrical interference. The transducer is also sensitive to distortions caused by interfering resonance between elements of the assembly, and accordingly must be physically supported without reducing its performance.
These prods are subjected to significant force and wear in the course of normal use. The device must be able, at least, to withstand the force of the weight of the operator. The direction of the force will often result in significant flexure of the rod. Good connection between the transducer and the rod, however, is essential. This connection must withstand any flexure without any separation, or the detection will not be reliable. In the design proposed in the prior art, the transducer is mechanically coupled by gluing the crystal to a disk-shaped ceramic insulator, which is in turn glued to the coupling end of the rod. This design has not proved reliable. The coupling is susceptible to damage. The glue, which must form the structural connection as well as the acoustic connection, is particularly subject to shear due to shock and vibration, as well as shifting induced by thermal effects and other disturbances. If the device is dropped, for instance, no structural lateral support is provided to the transducer or the coupling. However, lateral support must be provided without dampening the acoustic transmission.
Further the prod itself is subject to significant physical wear as a digging tool. Under regular use, the simple prod (without acoustic sensitivity) has an expected lifetime of three months. The cost of replacing acoustic detectors at this rate is very high. However, the sensitivity of the acoustic coupling makes replacing the rod in the detector difficult. This replacement could not be made reliably in the field.